DC neutral-point voltage balance control device and method for AC / DC conversion system

Abstract

The invention relates to a DC neutral-point voltage balance control device and method for an AC / DC conversion system. The device comprises an inductor L, a capacitor C, a pair of semiconductor switches S1 and S2 which are connected in series, and a control module, wherein the capacitor C and a series circuit of the semiconductor switches S1 and S2 are connected in parallel with positive and negative electrodes of a DC bus of the AC / DC conversion system, one end of the inductor L is connected to a midpoint of the DC bus, and the other end of the inductor L is connected to a midpoint of the series circuit of the semiconductor switches S1 and S2; the control module generates a pair of complementary pulse control signals based on the ratio of the direct-current bus neutral-point voltage detected in real time to the direct-current bus positive and negative electrode voltage, controls the semiconductor switches S1 and S2 to be complementarily conducted, and gradually adjusts the direct-current bus neutral-point voltage to be balanced according to a set fixed step length. The number of required devices is small, the control method is simple, and neutral-point voltage balance of a single-machine or multi-machine parallel AC / DC conversion device can be achieved only through one hardware circuit.

Description

Technical Field

[0001] The present invention relates to the technical field of DC midpoint voltage balance in an AC / DC conversion system, and in particular to a device and method for controlling DC midpoint voltage balance in an AC / DC conversion system. Background Art

[0002] Three-level converters, with their numerous advantages, such as large capacity, high withstand voltage rating, low output current harmonics, and minimal switching losses, have become essential power electronic devices in new power systems. However, due to their numerous components and complex control strategies, three-level converters also present an unavoidable problem compared to two-level converters: midpoint voltage balance. During operation, three-level AC / DC converters must maintain midpoint voltage balance. Fluctuations in midpoint voltage can distort the output voltage and current waveforms, increase low-order harmonics at the output, and severely degrade power quality. Furthermore, excessive voltage across switching devices can reduce their lifespan or even damage them.

[0003] There are many methods for midpoint voltage balancing, mainly the following:

[0004] 1. Improve the modulation algorithm: Select a reasonable vector and its action time so that the amount of electricity flowing into and out of the midpoint voltage is balanced within one or several control cycles, thereby achieving the purpose of controlling the midpoint voltage. For example, CN119483258A discloses a midpoint voltage balance control system and method for a three-level Buck DC converter. This method uses a single-stage three-level Buck circuit but relies on a dual closed-loop PI controller to modulate both edges of the PWM wave. The control algorithm is complex and requires a large number of circuit parameters to be collected. This method is more difficult to control the midpoint voltage balance in scenarios with large modulation indexes or low power factors.

[0005] 2. Add switching device hardware circuits at both ends of the DC-side capacitors to balance the voltages on the upper and lower DC-side capacitors through the action of the switching devices. For example, CN112909919B discloses a two-stage, three-level, three-phase, four-wire energy storage converter midpoint voltage balancing method and system. This method uses a cascade topology with a three-level Buck / Boost circuit as the front stage and a T-type three-level inverter circuit as the back stage. This method requires the introduction of multiple sets of switching transistors, inductors, and complex filtering structures, resulting in high hardware costs and a complex control algorithm.

[0006] 3. The DC side uses multiple independent DC power supplies to directly control the midpoint voltage at a fixed value. This method requires adding a large amount of additional hardware circuits, resulting in increased circuit costs.

[0007] Therefore, it is of great significance to adopt a method that is both economical, practical and simple to control in order to balance the midpoint voltage of the AC / DC DC side. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a DC midpoint voltage balance control device and method for an AC / DC conversion system, which can be applied to AC / DC conversion devices of a single machine or multiple machines in parallel, requires a small number of components, and has a simple control method.

[0009] The technical solution adopted by the present invention to solve its technical problems is as follows: providing a DC midpoint voltage balance control device for an AC / DC conversion system, comprising a first semiconductor switch, a second semiconductor switch, an inductor, a capacitor, and a control module; one end of the inductor is connected to the midpoint of the DC bus of the AC / DC conversion system, and the other end is electrically connected to the source of the first semiconductor switch and the drain of the second semiconductor switch; the drain of the first semiconductor switch is electrically connected to the positive electrode of the DC bus and one end of the capacitor, respectively, and the source of the second semiconductor switch is electrically connected to the negative electrode of the DC bus and the other end of the capacitor, respectively; the control module is configured to generate a pulse control signal based on the real-time collected DC bus midpoint voltage and the DC bus positive and negative voltages to control the complementary conduction of the first semiconductor switch and the second semiconductor switch, thereby gradually adjusting the DC bus midpoint voltage balance of the AC / DC conversion system with a fixed step size.

[0010] Furthermore, the step of generating a pulse control signal based on the real-time collected DC bus midpoint voltage and DC bus positive and negative voltages includes:

[0011] The ratio between the DC bus midpoint voltage and the DC bus positive and negative voltages is used as a modulation signal;

[0012] The amplitude of the modulation signal at the current moment is calculated based on the DC bus midpoint voltage and the DC bus positive and negative voltages collected in real time;

[0013] Comparing the amplitude of the modulation signal at the current moment with the midpoint of the amplitude of the carrier signal, and adjusting the target amplitude of the modulation signal at the next moment with a fixed step size according to the comparison result;

[0014] The target amplitude of the modulation signal is compared with the amplitude of the carrier signal to generate the pulse control signal.

[0015] Furthermore, the carrier signal is a triangular carrier signal.

[0016] Furthermore, the pulse control signal includes:

[0017] a first pulse control signal connected to the gate of the first semiconductor switch, controlling the first semiconductor switch to turn on when the triangular carrier signal is at a falling edge and its amplitude is equal to the target amplitude of the modulation signal, and controlling the first semiconductor switch to turn off when the triangular carrier signal is at a rising edge and its amplitude is equal to the target amplitude of the modulation signal;

[0018] a second pulse control signal connected to the gate of the second semiconductor switch, controlling the second semiconductor switch to turn on when the triangular carrier signal is at a rising edge and its amplitude is equal to the target amplitude of the modulation signal, and controlling the second semiconductor switch to turn off when the triangular carrier signal is at a falling edge and its amplitude is equal to the target amplitude of the modulation signal.

[0019] Furthermore, adjusting the target amplitude of the modulation signal at the next moment with a fixed step size according to the comparison result includes:

[0020] If the amplitude of the modulation signal at the current moment is greater than the midpoint of the amplitude of the carrier signal, then setting the target amplitude of the modulation signal at the next moment to be the amplitude of the modulation signal at the current moment minus the fixed step size;

[0021] If the amplitude of the modulation signal at the current moment is less than the midpoint of the amplitude of the carrier signal, then setting the target amplitude of the modulation signal at the next moment to be the amplitude of the modulation signal at the current moment plus the fixed step size;

[0022] If the amplitude of the modulation signal at the current moment is equal to the midpoint of the amplitude of the carrier signal, the target amplitude of the modulation signal at the next moment is set to the amplitude of the modulation signal at the current moment.

[0023] Furthermore, the AC / DC conversion system includes a plurality of AC / DC converters connected in parallel.

[0024] The present invention also provides a method for controlling the DC midpoint voltage balance of an AC / DC conversion system, which is applied to the above-mentioned device and includes the following steps:

[0025] Collect the DC bus midpoint voltage and DC bus positive and negative voltages of the AC / DC conversion system;

[0026] The ratio between the DC bus midpoint voltage and the DC bus positive and negative pole voltages is controlled to be one half.

[0027] Furthermore, controlling the ratio between the DC bus midpoint voltage and the DC bus positive and negative pole voltages to be one-half includes:

[0028] The ratio between the DC bus midpoint voltage and the DC bus positive and negative voltages is used as a modulation signal;

[0029] The amplitude of the modulation signal at the current moment is calculated based on the DC bus midpoint voltage and the DC bus positive and negative voltages collected in real time;

[0030] Comparing the amplitude of the modulation signal at the current moment with the midpoint of the amplitude of the carrier signal, and adjusting the target amplitude of the modulation signal at the next moment with a fixed step size according to the comparison result;

[0031] The target amplitude of the modulation signal is compared with the amplitude of the carrier signal, and a pulse control signal is generated to control the first semiconductor switch and the second semiconductor switch to be complementary conductive.

[0032] Furthermore, adjusting the target amplitude of the modulation signal at the next moment with a fixed step size according to the comparison result includes:

[0033] If the amplitude of the modulation signal at the current moment is greater than the midpoint of the amplitude of the carrier signal, then setting the target amplitude of the modulation signal at the next moment to be the amplitude of the modulation signal at the current moment minus the fixed step size;

[0034] If the amplitude of the modulation signal at the current moment is less than the midpoint of the amplitude of the carrier signal, then setting the target amplitude of the modulation signal at the next moment to be the amplitude of the modulation signal at the current moment plus the fixed step size;

[0035] If the amplitude of the modulation signal at the current moment is equal to the midpoint of the amplitude of the carrier signal, the target amplitude of the modulation signal at the next moment is set to the amplitude of the modulation signal at the current moment.

[0036] Furthermore, the carrier signal is a triangular carrier signal.

[0037] Furthermore, the controlling the first semiconductor switch and the second semiconductor switch to be complementary conductive includes:

[0038] When the triangular carrier signal is at a falling edge and its amplitude is equal to the target amplitude of the modulation signal, controlling the first semiconductor switch to be turned on and the second semiconductor switch to be turned off;

[0039] When the triangular carrier signal is at a rising edge and its amplitude is equal to the target amplitude of the modulation signal, the first semiconductor switch is controlled to be turned off and the second semiconductor switch is controlled to be turned on.

[0040] Furthermore, the frequency of the triangular carrier signal is fixed.

[0041] Beneficial effects

[0042] Due to the adoption of the above-mentioned technical solution, the present invention has the following advantages and positive effects compared with the prior art: the present invention generates a pulse control signal based on the real-time collected DC bus midpoint voltage and the DC bus positive and negative pole voltages to control the complementary conduction of the semiconductor switch S1 and the semiconductor switch S2, and gradually adjusts the DC bus midpoint voltage balance of the AC / DC conversion system with a set fixed step size, which can effectively solve the DC side midpoint voltage offset of the AC / DC conversion device caused by unbalanced AC load or unbalanced source and load of the positive and negative poles on the DC side. The number of devices is small and the control method is simple. It is particularly suitable for microgrid, energy storage, and flexible DC interconnection scenarios constructed by three-level AC / DC conversion devices to ensure stable operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 1 is a structural diagram of a DC midpoint voltage balancing circuit according to a first embodiment of the present invention;

[0044] Figure 2 1 is a schematic diagram of the pulse control signal generation principle of the first and second embodiments of the present invention;

[0045] Figure 3 This is a flow chart of the DC neutral voltage balance control algorithm of the first and second embodiments of the present invention. DETAILED DESCRIPTION

[0046] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0047] A first embodiment of the present invention relates to a DC bus midpoint voltage balancing control device for an AC / DC conversion system, comprising:

[0048] The hardware circuit has a structure like Figure 1 As shown, it consists of two fully controlled semiconductor switches S1 and S2, an inductor L, and a capacitor C: the semiconductor switches S1 and S2 are connected in series and then connected in parallel between the positive electrode P and the negative electrode N of the DC bus of the AC / DC conversion system; the capacitor C is connected in parallel with the series circuit of the semiconductor switches S1 and S2, and one end of the inductor L is connected to the midpoint of the series circuit of the semiconductor switches S1 and S2, and the other end is connected to the midpoint O of the DC bus of the AC / DC conversion system;

[0049] Control module, based on real-time detection of the midpoint voltage of the AC / DC conversion system and DC bus voltage The ratio between them generates a pair of complementary pulse control signals, which are respectively connected to the gates of semiconductor switch S1 and semiconductor switch S2 to control the complementary conduction of the two to balance the DC bus midpoint voltage of the AC / DC conversion system.

[0050] Among them, the pulse control signal can be generated by pulse width modulation, and the midpoint voltage and DC bus voltage The ratio between them is used as the modulation wave U ref , compared with the triangular carrier signal, and a pulse control signal is generated according to the comparison result.

[0051] More specifically, define is the magnitude of the modulation wave at time n, is the amplitude of the triangular carrier signal (i.e., peak value), is the DC bus positive and negative voltage of the AC / DC conversion system, is the lower capacitor voltage of the AC / DC conversion system, The fixed step size of the modulation wave is adjusted for each control cycle. To adjust for / 2, that is, the lower capacitor voltage of the AC / DC conversion system can be made half of the DC bus positive and negative voltages, that is, the DC bus midpoint voltage of the AC / DC conversion system is kept balanced.

[0052] like Figure 2 As shown, by comparing the modulated wave with the triangular carrier signal in real time, the pulse control signals of S1 and S2 can be obtained. The specific generation method is as follows:

[0053] When the triangular carrier is on the rising edge and its magnitude is equal to that of the modulation wave, S2 is controlled to be turned on and S1 is controlled to be turned off. At this time, the inductor L, the semiconductor switch S2 and the lower capacitor of the AC / DC conversion system form a current loop, and the lower capacitor discharges.

[0054] When the triangular carrier is on the falling edge and its size is equal to the modulation wave, S1 is controlled to be turned on and S2 is controlled to be turned off. At this time, the inductor L, semiconductor switch S1, capacitor C and the lower capacitor of the AC / DC conversion system form a current loop, and the lower capacitor is charged.

[0055] Specific control methods such as Figure 3 As shown:

[0056] Step 1: Calculate the current modulation signal size: ;

[0057] Step 2: Since Uref is calibrated between 0 and 1, the triangle carrier amplitude can be set to 1. and Compare the triangular carrier signal to compare:

[0058] like , the modulation wave needs to be controlled to gradually decrease, that is ;

[0059] like , the modulation wave needs to be controlled to increase gradually, that is ;

[0060] like , the modulation wave needs to be controlled to remain unchanged, that is ;

[0061] Step 3: Set the target amplitude of the modulation wave signal at the next moment The triangular carrier signal is compared to generate pulse control signals for semiconductor switches S1 and S2 to control their on and off and balance the DC side midpoint voltage.

[0062] A second embodiment of the present invention relates to a method for controlling DC midpoint voltage balance in an AC / DC conversion system, which is applied to the above-mentioned device and includes the following steps:

[0063] Collect the DC bus midpoint voltage and DC bus positive and negative voltages of the AC / DC conversion system;

[0064] A pulse control signal is generated based on the ratio between the DC bus midpoint voltage and the DC bus positive and negative voltages to control the complementary conduction of semiconductor switches S1 and S2 to balance the DC bus midpoint voltage of the AC / DC conversion system.

[0065] Among them, the pulse control signal can be generated by pulse width modulation, and the midpoint voltage and DC bus voltage The ratio between them is used as the modulation wave U ref , compared with the triangular carrier signal, and a pulse control signal is generated according to the comparison result.

[0066] More specifically, define is the magnitude of the modulation wave at time n, is the amplitude of the triangular carrier signal (i.e., peak value), is the DC bus positive and negative voltage of the AC / DC conversion system, is the lower capacitor voltage of the AC / DC conversion system, The fixed step size of the modulation wave is adjusted for each control cycle. for / 2, that is, the lower capacitor voltage of the AC / DC conversion system can be made half of the DC bus positive and negative voltages, that is, the DC bus midpoint voltage of the AC / DC conversion system is kept balanced.

[0067] As Figure 2 shown, the modulation wave is compared with the triangular carrier signal in real time to obtain the pulse control signals of S1 and S2, and the specific generation method is as follows:

[0068] When the triangular carrier is on the rising edge and the size is equal to the modulation wave, S2 is turned on and S1 is turned off; at this time, the inductor L, the semiconductor switch S2 and the lower capacitor of the AC / DC conversion system form a current loop, and the lower capacitor is discharged.

[0069] When the triangular carrier is on the falling edge and the size is equal to the modulation wave, S1 is turned on and S2 is turned off; at this time, the inductor L, the semiconductor switch S1, the capacitor C and the lower capacitor of the AC / DC conversion system form a current loop, and the lower capacitor is charged.

[0070] The specific control method is shown in Figure 3 .

[0071] Step 1: calculate the size of the modulation wave signal at the current time as .

[0072] Step 2: compare with to generate pulse control signals to gradually adjust to .

[0073] If , the modulation wave needs to be gradually reduced, that is, .

[0074] If , the modulation wave needs to be gradually increased, that is, .

[0075] If , the modulation wave needs to be kept unchanged, that is, .

[0076] Step 3: compare the target amplitude size of the modulation wave signal at the next time with the triangular carrier signal to generate pulse control signals of the semiconductor switches S1 and S2 to control their conduction and turn-off, and balance the midpoint voltage of the DC side.

Claims

1. A DC midpoint voltage balance control device for an AC / DC conversion system, characterized in that: The device comprises a first semiconductor switch, a second semiconductor switch, an inductor, a capacitor, and a control module; one end of the inductor is connected to the midpoint of the DC bus of the AC / DC conversion system, and the other end is electrically connected to the source of the first semiconductor switch and the drain of the second semiconductor switch; the drain of the first semiconductor switch is electrically connected to the positive electrode of the DC bus and one end of the capacitor, respectively, and the source of the second semiconductor switch is electrically connected to the negative electrode of the DC bus and the other end of the capacitor, respectively; the control module is configured to generate a pulse control signal based on the real-time collected DC bus midpoint voltage and the DC bus positive and negative voltages to control the complementary conduction of the first semiconductor switch and the second semiconductor switch, thereby gradually adjusting the DC bus midpoint voltage balance of the AC / DC conversion system with a fixed step size.

2. The device according to claim 1, characterized in that Generating a pulse control signal according to the real-time collected DC bus midpoint voltage and DC bus positive and negative voltages includes: The ratio between the DC bus midpoint voltage and the DC bus positive and negative voltages is used as a modulation signal; The amplitude of the modulation signal at the current moment is calculated based on the DC bus midpoint voltage and the DC bus positive and negative voltages collected in real time; Comparing the amplitude of the modulation signal at the current moment with the midpoint of the amplitude of the carrier signal, and adjusting the target amplitude of the modulation signal at the next moment with a fixed step size according to the comparison result; The target amplitude of the modulation signal is compared with the amplitude of the carrier signal to generate the pulse control signal.

3. The device according to claim 2, characterized in that The carrier signal is a triangular carrier signal, and the pulse control signal includes: a first pulse control signal connected to the gate of the first semiconductor switch, controlling the first semiconductor switch to turn on when the triangular carrier signal is at a falling edge and its amplitude is equal to the target amplitude of the modulation signal, and controlling the first semiconductor switch to turn off when the triangular carrier signal is at a rising edge and its amplitude is equal to the target amplitude of the modulation signal; a second pulse control signal connected to the gate of the second semiconductor switch, controlling the second semiconductor switch to turn on when the triangular carrier signal is at a rising edge and its amplitude is equal to the target amplitude of the modulation signal, and controlling the second semiconductor switch to turn off when the triangular carrier signal is at a falling edge and its amplitude is equal to the target amplitude of the modulation signal.

4. The device according to claim 2, characterized in that The step of adjusting the target amplitude of the modulation signal at the next moment with a fixed step size according to the comparison result includes: If the amplitude of the modulation signal at the current moment is greater than the midpoint of the amplitude of the carrier signal, then setting the target amplitude of the modulation signal at the next moment to be the amplitude of the modulation signal at the current moment minus the fixed step size; If the amplitude of the modulation signal at the current moment is less than the midpoint of the amplitude of the carrier signal, then setting the target amplitude of the modulation signal at the next moment to be the amplitude of the modulation signal at the current moment plus the fixed step size; If the amplitude of the modulation signal at the current moment is equal to the midpoint of the amplitude of the carrier signal, the target amplitude of the modulation signal at the next moment is set to the amplitude of the modulation signal at the current moment.

5. The device according to claim 1, characterized in that The AC / DC conversion system includes a plurality of AC / DC converters connected in parallel.

6. A DC midpoint voltage balance control method for an AC / DC conversion system, characterized in that: The device according to any one of claims 1 to 5 comprises the following steps: Collect the DC bus midpoint voltage and DC bus positive and negative voltages of the AC / DC conversion system; The ratio between the DC bus midpoint voltage and the DC bus positive and negative pole voltages is controlled to be one half.

7. The method according to claim 6, characterized in that The controlling the ratio between the DC bus midpoint voltage and the DC bus positive and negative pole voltages to be one-half includes: The ratio between the DC bus midpoint voltage and the DC bus positive and negative voltages is used as a modulation signal; The amplitude of the modulation signal at the current moment is calculated based on the DC bus midpoint voltage and the DC bus positive and negative voltages collected in real time; Comparing the amplitude of the modulation signal at the current moment with the midpoint of the amplitude of the carrier signal, and adjusting the target amplitude of the modulation signal at the next moment with a fixed step size according to the comparison result; The target amplitude of the modulation signal is compared with the amplitude of the carrier signal, and a pulse control signal is generated to control the first semiconductor switch and the second semiconductor switch to be complementary conductive.

8. The method according to claim 7, characterized in that The step of adjusting the target amplitude of the modulation signal at the next moment with a fixed step size according to the comparison result includes: If the amplitude of the modulation signal at the current moment is greater than the midpoint of the amplitude of the carrier signal, then setting the target amplitude of the modulation signal at the next moment to be the amplitude of the modulation signal at the current moment minus the fixed step size; If the amplitude of the modulation signal at the current moment is less than the midpoint of the amplitude of the carrier signal, then setting the target amplitude of the modulation signal at the next moment to be the amplitude of the modulation signal at the current moment plus the fixed step size; If the amplitude of the modulation signal at the current moment is equal to the midpoint of the amplitude of the carrier signal, the target amplitude of the modulation signal at the next moment is set to the amplitude of the modulation signal at the current moment.

9. The method according to claim 7, characterized in that The carrier signal is a triangular carrier signal.

10. The method according to claim 9, characterized in that The controlling the first semiconductor switch and the second semiconductor switch to be complementary conductive includes: When the triangular carrier signal is at a falling edge and its amplitude is equal to the target amplitude of the modulation signal, controlling the first semiconductor switch to be turned on and the second semiconductor switch to be turned off; When the triangular carrier signal is at a rising edge and its amplitude is equal to the target amplitude of the modulation signal, the first semiconductor switch is controlled to be turned off and the second semiconductor switch is controlled to be turned on.

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